1. Field of the Invention
This invention relates to position location systems that utilize wireless signals to determine the location of an electronic mobile device.
2. Description of Related Art
Existing position location technologies based on GPS use a network of satellites in earth orbit that transmit signals at a known time. A GPS receiver on the ground measures the time-of-arrival of the signals from each satellite in the sky that it can “see”. The time-of-arrival of the signal along with the exact location of the satellites and the exact time the signal was transmitted from each satellite is used to triangulate the position of the GPS receiver. A GPS receiver requires four satellites to make a triangulation and the performance of the resulting position location increases as the number of satellites that can be detected increases.
One problem with GPS-based position location determination arises if only three (or less) satellites can be found, and in such an instance (and in the absence of other ancillary information) it is not possible to accurately locate the GPS receiver. For example, if the GPS receiver's view of the sky is obstructed (e.g., deep inside a concrete building) it may not be possible to obtain enough GPS measurements to determine receiver location.
For a wireless communication receiver (i.e. mobile station), the existing wireless network of base stations can be used for position location purposes in a similar manner as the network of GPS satellites for GPS receiver. Theoretically-speaking, the exact location of each base station, the exact time at which the base station is transmitting, and the time-of-arrival of the base station's signal at a mobile station (e.g., cell phone) can be used to trilaterate the position of the mobile station. This technique is referred to as Advanced Forward Link Trilateration (AFLT).
The AFLT method may be used by itself for position location purposes; alternatively, in order to enhance performance of a GPS system, the existing network of wireless communication base stations can be treated as a secondary network of “satellites” for position location purposes in a GPS-capable mobile station (i.e., a device that includes both GPS and wireless communication receivers). The AFLT technique, combined with GPS algorithms, is referred to as hybrid, or Assisted-GPS (A-GPS).
AFLT is a method for determining the position of a mobile station using a plurality of wireless communication network base stations each emitting a unique pilot signal. The AFLT method includes taking a plurality of data measurements of the pilot signals from each of the plurality of base stations. Each of the data measurements includes an earliest time-of-arrival estimate for each pilot signal. In some embodiments, the data measurements further include an RMSE estimate, time of measurement for each time-of-arrival, and an energy measurement (e.g., Ec/Io) for all resolvable paths of the pilot signal.
The data measurements obtained by the AFLT algorithm may be used alone to determine mobile station position; alternatively one or more of the representative AFLT measurements may be used together with representative GPS measurements to determine the position of the mobile station. In some embodiments the mobile station comprises a cell phone and the method further comprises wirelessly connecting the cell phone to one of the cellular base stations prior to taking data; the base station provides a cell search list to the cell phone of all cellular base stations in the area from which data measurements may be taken. In embodiments that include a GPS system, a base station can also provide a GPS search list, which can be used to reduce the time necessary for the mobile station to perform the GPS search and thus to reduce time-to-fix.
In practice, AFLT (including A-GPS) has proven to be of only limited success for position location purposes, in part because repeaters employed in wireless networks cause an ambiguity as to the point of transmission of the pilot signal. In other words, a mobile station cannot currently distinguish whether a received signal was transmitted from the donor Base Transceiver Station (BTS) or the repeater. In such a circumstance, the AFLT measurement cannot be used to accurately determine position because the point of transmission of the pilot signal is unknown (e.g., it is unknown whether it was transmitted directly from a donor BTS or repeated through a repeater). In addition, the repeater will also have internal delays, typically in a range from hundreds of nanoseconds up to tens of microseconds, potentially resulting in a position location error in the range of about 24.4 meters (for 100 nanoseconds) to about 2.44 kilometers (for 10 microseconds).
One proposed solution to the problem of determining location in the presence of repeaters is to simply exclude all AFLT measurements whenever the mobile station is in an area where repeaters are present. However, this solution completely precludes AFLT position location in those areas, and also prevents any AFLT portion of A-GPS from being utilized whenever repeaters are present, thus reducing position location availability and yield, and increasing GPS search windows, resulting in longer times-to-fix.
It has been suggested to introduce a signature on the reverse link, as described in U.S. Pat. No. 6,501,955 in order to help position determination. Unfortunately, the RL signature is likely to be of only limited assistance in mitigating the effects of repeaters on position location, because the mobile station uses AFLT measurements from the forward link (rather than the reverse link) for position location. Since there is no guarantee that the forward link back to the mobile station will follow the same path as the reverse link from the mobile station (i.e., through the same repeater), the reverse link signature is likely to be suboptimal for identifying repeater information for position location purposes. Introducing a signature onto the FL signal has also been suggested, such as described in U.S. Pat. No. 6,501,955, however no practical solution has yet been developed.